How to write the python for this complicated situation ?

How are you,Everyone!

I research about the network distance from post box points to neighbourhood points.

1.The distance from post box point to neighbourhood point is (D).
2. For each post box,I just consider the neighbourhood points within 800 meters roadnetwork distance.
3. The population of neighbourhood is (P).

There is a fomular to calculate the capasity of each post box points : ∑={(D1)/800}*P1+{(D2)/800}*P2+…… +{(Dn)/800}*Pn


I have a database, which contains the roadnetwork, neighbourhood points, post box points,neighbourhood population. Now,I wanna get the sum ∑ by using the python, how should I write it?

Many thanks!